Sendmail is a Mail Transfer Agent, which is the
program that moves mail from one machine to
another. Sendmail implements a general
internetwork mail routing facility, featuring
aliasing and forwarding, automatic routing to
network gateways, and flexible configuration.

Kolab is a groupware system that provides a
comprehensive and flexible solution for
workgroups. Email, contacts, and group calendars
can be managed via the Web, Windows/Outlook, and
Linux/KDE. It supports offline synchronization and
full seamless support of mixed client environments
because the Kolab-XML storage format is fully open
and avoids MAPI/TNEF.

The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, Webmail, and mailing list services within a single consistent framework. Individual components can be enabled or disabled at will. Courier implements basic Web-based calendaring and scheduling services integrated in the Webmail module.

IndiMail is a messaging platform built using qmail and other messaging software. IndiMail has the ability to distribute the users in a domain across multiple hosts. IndiMail has multiple queues (called a queue collection), which can achiever very high concurrencies. The system IDs and configuration files used by IndiMail are all configurable through a set of environment variables, allowing you to customize IndiMail the way you want. IndiMail does not use hard-coded uid/gid in the executables, which makes it easy to distribute a binary package. IndiMail has features that any modern email server should have: SMTP, QMQP, QMTP, IMAP, POP3, DKIM, BATV, greylisting, filtering capability, and spam/virus control.

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. In style it is similar to Smail 3, but its facilities are much more extensive. Exim is in use at many sites around the world.

DJIGZO Email Encryption Gateway is a centrally managed email server that encrypts and decrypts incoming and outgoing email at the gateway level. Djigzo currently supports two encryption standards: S/MIME and PDF encryption. The built-in Data Leak Prevention (DLP) module can be used to prevent certain information from leaving the organization via email. DLP can be configured to filter on credit card numbers, bank account numbers, excessive amounts of email addresses or other personal information in one email message, and more. DJIGZO can be installed on most Linux and Unix based systems. Installation packages are available for Ubuntu, Debian, Red Hat and CentOS. A ready to run virtual appliance for VMware and Hyper-V is available. An add-on for BlackBerry is available.

getmail is intended as a simple, secure, and
reliable replacement for fetchmail. It retrieves
email (either all messages, or only unread
messages) from one or more POP3, SPDS, or IMAP4
servers (with or without SSL) for one or more
email accounts, and reliably delivers into
qmail-style Maildirs, mboxrd files, or through
external MDAs (command deliveries) specified on a
per-account basis. getmail also has excellent
support for domain (multidrop) mailboxes,
including delivering messages to different users
or destinations based on the envelope recipient
address.

MailCleaner is an anti-spam/anti-virus SMTP
gateway with user and admin
Web interfaces, a quarantine system, and support
for multiple domains,
templates, and languages. It's based on Debian and
uses Bayes, RBLs,
Spamassassin, MailScanner, and ClamAV.